1. Field of the Invention
The present invention relates to a current roller for an electrolytic strip coating plant.
2. Description of the Related Art
A current roller generally has the purpose of returning the current transferred from coating anodes to a strip back to a rectifier. A distinction is made between current roller arrangements for horizontal and vertical strip travel.
In the case of horizontal coating systems, the strip to be coated is guided horizontally through a coating cell, wherein the current rollers are arranged in front of and behind the cell, rest from the top or bottom against the strip and apply a linear pressure to the strip by means of a rubber-coated counter-roller. When the circumferential speed of the roller corresponds to the strip speed, the current is transmitted from the strip to the current roller as a result of the linear contact between the strip and the current roller and the current is returned to the rectifier from the current roller by means of slip ring systems.
In vertical coating systems, the strip to be coated is deflected by 180xc2x0 after emerging from the coating cell by means of current rollers which simultaneously serve as guide rollers, so that the strip can enter the next following coating cell. As a result of the contact surface between the strip and the current roller, the current flows from the strip to the current roller and from there through slip ring systems back to the rectifier. In this case, the current roller not only serves to transmit current, but also to guide the strip. Because of the bending stiffness of the strips, but also because of the relatively large distances between the coating cells, the current roller for vertical plants must be constructed with a relatively large diameter.
Because of the internal electrical resistance, the current roller is heated and its diameter increases accordingly. The strip surrounding the current roller is another heat source which is also heated as a result of the internal electrical resistance. In the case of strip thicknesses in the range of below 1 mm, the strip temperature may reach values of above 100xc2x0 C. and the current roller is heated accordingly.
For these reasons, current rollers are usually provided with an internal cooling system which has the purpose of ensuring a uniform temperature distribution over the current roller length. The purpose of this is to ensure that sections having different temperatures do not result in different diameters over the length of the roller body. This would have the consequence that the strip guidance is no longer ensured and experience has shown that this also leads to a quality reduction of the coated surfaces.
Various embodiments of current roller cooling systems are known in the art.
In one embodiment, the current roller is essentially constructed as a round hollow body whose outer wall is of an acid-proof and current-conducting metal. The hollow body is filled partially or completely with cooling water, wherein cold water is introduced through one roller neck and heated water is discharged through the other roller neck. This solution is the least expensive, but has the following disadvantages:
The slow flow speed of the cooling water toward the inner wall surface means that the heat transfer is low when the roller is completely filled. In addition, the additional quantity of water increases the flywheel effect of the roller and, thus impairs the drive control. Also, a uniform temperature distribution over the roller body length cannot be ensured.
In accordance with another known solution, the hollow space is filled out substantially with displacement bodies, so that the latent water quantity in the interior of the current roller is reduced, however, the problem of the uncontrollable cooling effect remains.
The disadvantages described above are eliminated in accordance with another solution. In that case, tightly wound copper pipes are placed in a concentric gap between the outer current roller wall of high-grade steel and an inner cylinder of normal steel, wherein the cooling water flows with a high flow speed through the copper pipes. For effecting the heat transfer between the outer wall and the copper pipes, the remaining hollow space is filled by casting zinc metal into the hollow space.
This solution does ensure a controllable and uniform heat discharge, however, the manufacturing costs are significantly higher than in the solutions described previously. Another disadvantage is the fact that, after the roll wall has been ground repeatedly because of wear, the roller can only be repaired in a special shop, for example, by manufacturing a new wall and shrinking it onto the old wall which has previously been turned to size. A simple exchange of the used roller wall is not possible. Consequently, the operators of coating plants must have a large number of expensive current rollers in storage.
Therefore, it is the primary object of the present invention to provide a current roller which does not have the disadvantages discussed above, can be manufactured inexpensively and in an uncomplicated manner, and makes it unnecessary to keep complete rollers in storage.
In accordance with the present invention, a current roller having a roller axis for an electrolytical strip coating plant includes a roller wall having an inner wall surface and an outer wall surface and two essentially cylindrical base bodies having cylindrical walls and filling out the roller wall, wherein the base bodies have sides facing each other and facing away from each other, wherein the roller wall and the base bodies are releasably connected to each other.
As a result of the configuration according to the present invention, the roller wall can be easily exchanged, so that it is no longer necessary to keep entire current rollers in storage.
The current roller can be manufactured particularly inexpensively if the base bodies are made of normal steel and the roller wall of an acid-proof and electrically conductive material, such as high-grade steel.
If the cylinder walls of the base bodies are constructed so as to be electrically insulating, it is easier to control the current conduction in the roller wall. This makes it possible to ensure a uniform heating of the current roller. The cylinder walls can be electrically insulated, for example, by providing them with a hard, acid-proof, electrically insulating layer.
By providing on the inner wall surface of the roll wall an inner ring extending concentrically around the roller axis, the connectability between the roller wall and the base bodies can be effected especially simply. On the other hand, together with the electrical insulation of the cylinder walls, a completely symmetrical current conduction in the roller wall can be achieved.
The releasable fastening of the roller wall and the base bodies is particularly simple if the base bodies have throughbores which correspond to each other and extend parallel to the roller axis, wherein the throughbores receive fastening elements, for example, pull rods.